1. Field of the Invention
The invention relates to a method and system for etching a substrate. In particular, the invention relates to a method and system for etching large, high aspect ratio features in a substrate, such as those in micro-electromechanical devices (MEMs).
2. Description of Related Art
During semiconductor processing, plasma is often utilized to assist etch processes by facilitating the anisotropic removal of material along fine lines or within vias (or contacts) patterned on a semiconductor substrate. Examples of such plasma assisted etching include: (1) reactive etching with reactive radicals formed by plasma dissociation; (2) reactive ion etching (RIE) with reactive ions; and (3) ion assisted reactive etching with ions and radicals. In order to produce anisotropic features, the etching process must fall into one of the latter two categories.
In plasma assisted etching, fine patterns formed in a thin lithographic mask layer are transferred to an underlying layer on the substrate. For example, the lithographic mask layer is formed by applying a thin layer of radiation-sensitive material, such as photoresist, to an upper surface of the substrate, and exposing the radiation-sensitive material to a pattern of electromagnetic radiation. Thereafter, the fine pattern formed in the mask layer is transferred to the underlying thin film on the substrate during etching.
However, conventional plasma processes utilize electropositive plasma discharge (i.e., ion-electron plasma). In electropositive plasma, the region between the outermost portion of the plasma which has substantially equal numbers of ions and electrons over any arbitrary incremental volume thereof and a plasma confining surface (that causes a boundary condition) may be referred to as the “plasma sheath”. The thickness of the plasma sheath is on the order of the Debye length which corresponds to the characteristic distance over which significant charge densities may spontaneously exist.
For an electropositive plasma, the ion-electron plasma Debye length (λd,ie) may be expressed as:λd,ie=[(∈oTe)/(neq)]1/2,  (1)
where ∈o is the permittivity of free space, q is the charge of an electron, Te is the electron temperature, and ne is the electron density. For example, electropositive plasma having an electron temperature of Te˜1 eV and an electron density of ne˜1×1012/cm3, the Debye length is approximately 7.4 microns.
When etching large, high aspect ratio features, such as those encountered during processing of micro-electromechanical (MEM) devices, the sheath thickness (or Debye length) becomes about equal to or less than the transverse dimension of the feature being etched. As a result, the sheath edge is perturbed while etching the feature and the bulk plasma presses towards the interior of the feature.
For example, FIGS. 1A and 1B illustrate an etching process for a large, high aspect ratio feature using electropositive plasma. A feature 20 is etched into a substrate 10, wherein an ion flux 30 passes through plasma sheath 32, adjacent substrate 10, and assists the etching chemistry at the bottom of the feature 20. As shown in FIG. 1A, the transverse dimension 22 of feature 20 is larger than the thickness 34 of plasma sheath 32 and, therefore, the edge of plasma sheath 32 is perturbed as plasma enters feature 20. Consequently, the ion trajectories of ion flux 30 are varied due to the perturbation of plasma sheath 32 into feature 20, and some of these ion trajectories strike the sidewalls of feature 20. This bending of ion trajectories causes non-anisotropic etching of feature 20.
At present, one approach to counter this effect is the well-known Bosch process. In the Bosch process, pseudo-anisotropic features are produced by a two-step process that includes alternatingly and repeatedly performing an etching process followed by a film forming process. However, the Bosch process results in a limited etch rate and the formation of undulations 26 (see FIG. 1B) due to the alternating etching and film forming steps. Further, if the etch rate is increased by prolonging the time period for the etching step, the size of undulations 26 worsens.